Optical Proximity Correction (OPC) enables low-k1 lithography by changing the pattern shapes on the mask to correct for the non-linear process of printing those patterns on a wafer. OPC has continually improved in accuracy over the years by adding more physically based lithography models. Some other design and verification processes such as design for manufacture may also employ lithography models. Lithography models, including rigorous first principle models and fast approximate models, require calibration using measured data. The calibration of lithography models involves function evaluations and data fitting. There are two major concerns for the model calibration. First, the computational cost of function evaluations, especially for complex negative tone developer (NTD) resist models, is very high and dominates the calibration run time. Second, the fitting objective is a multi-modal function of the tuning parameters, and users are generally interested in models other than the model with the best fitting objective. As the model complexity increases, it is challenging to find a convergent solution.